In the simplest form, an organic electroluminescent (EL) device is comprised of an anode for hole injection, a cathode for electron injection, and an organic media sandwiched between these electrodes to support charge recombination that yields emission of light. These devices are also commonly referred to as organic light emitting diodes, or OLEDs. A basic organic EL element is described in U.S. Pat. No. 4,356,429. In order to construct a pixelated OLED display such as is useful as a display such as, for example, a television, computer monitor, cell phone display, or digital camera display, individual organic EL elements can be arranged as pixels in a matrix pattern. These pixels can all be made to emit the same color, thereby producing a monochromatic display, or they can be made to produce multiple colors such as a red, green, blue (RGB) display. For purposes of this disclosure, a pixel is considered the smallest individual unit which can be independently stimulated to produce light. As such, the red pixel, the green pixel and the blue pixel are considered as three distinct pixels.
Color organic EL displays have also recently been described that are constructed as to have four differently colored pixels. One type of display having four differently colored pixels that are red, green, blue, and white in color is known as an RGBW design. Examples of such four pixel displays are shown in U.S. Pat. No. 6,771,028, U.S. Patent Application Publication US2002/0186214A1, and U.S. Patent Application Publication US2004/0113875A1. Such RGBW displays can be constructed using a white organic EL emitting layer with red, green, and blue color filters. The white pixel area is left unfiltered. This design has the advantage of lower power consumption and current density compared to a three-color filtered white-emitting organic EL displays by using the higher efficiency white pixels to produce a portion of gray scale colors.
Organic EL displays are sometimes driven with active matrix circuitry. Active matrix circuitry typically consists of active circuit components such as multiple transistors and one or more capacitors per pixel. Active matrix circuitry components also comprises signal lines such as power lines for supplying electric power to the pixels, data lines for supplying a voltage or current signal to adjust the brightness of the pixels, and select lines for sequentially activating a row of pixels thereby causing the pixels of each row to adjust in brightness in response to the signal of the data lines. The signal lines are typically shared by either a row or a column of pixels. These circuit components permit the pixels to remain illuminated even when the pixels are not being directly addressed. Examples of organic EL displays driven by active matrix circuitry are shown in U.S. Pat. Nos. 5,550,066, 6,281,634, and 6,456,013.
It is known that the efficiency of organic EL displays decreases over time with use. It is also known that the rate of decay in the efficiency is dependant on the electric current per unit of surface area (hereafter referred to as current density) applied to the organic light emitting diode. For multicolored organic EL displays, it is therefore often desirable to adjust size of the emitting areas of each the differently colored pixel so as to reduce the current density in pixels which are driven more frequently, are of lower efficiency, or are constructed of materials which decay more rapidly with increased current density. In such a configuration, for example, a low efficiency color pixel having a rapid rate of decay may be provided with a larger emitting area than a different colored pixel that is more efficient so that the desired brightness of the lower efficiency pixel can be achieved at a lower current density thereby increasing display lifetime. By adjusting the size of the emitting areas of the differently colored pixel, and thereby the current densities, the lifetimes of the organic EL elements in each pixel can thereby be balanced lessening the possibility that a particular colored pixel with decay more quickly than other colored pixels. Examples of organic EL displays having differently colored pixels where the emitting area of the differently color pixels has been optimized are shown in U.S. Pat. Nos. 6,366,025 and 6,747,618 and U.S. Patent Application Publications US2004/0164668A1 and US2004/0173819A1.
In an organic EL display, the pixels are arranged in a series of rows and columns. The simplest arrangement is a strip pattern where the pixels are aligned in both the row and column direction. In such a stripe arrangement, pixels having the same color may be aligned in the same direction such as the column direction. Alternate arrangements where the same colored pixels are not arranged in a row or column direction have also been shown. For example, the delta pattern, where the red, green, and blue pixels are arranged in a triangular fashion is known in the art. Examples of these various pixel arrangements may be found in U.S. Pat. Nos. 6,281,634, 6,456,013 and 6,768,482 and U.S. Patent Application Publication US2003/067458A1. Other patterns, such as a quad pattern have been shown which are particularly useful for displays having four differently colored pixels such as an RGBW display. In a quad pattern, four differently colored pixels are arranged in a rectangular fashion with each differently colored pixel disposed in a different corner of the rectangle. Examples of quad patterns are shown in U.S. Pat. No. 6,771,028.
When these various pixels patterns are coupled with the active matrix circuitry, the arrangement of the circuit components and signal lines typically must be adjusted to accommodate the pixel pattern. This is particularly true for displays having a configuration where the active matrix circuit components are located in a plane between the plane of the organic EL elements and the viewer. This is because, in this configuration, the circuit components and signal lines are typically opaque and would therefore block the light intended for the viewer.
As shown in U.S. Pat. No. 6,747,618, one approach to adjusting the size of the emitting areas of the differently colored pixels is to change the widths of the pixels while making the heights of the pixels the same. This approach is particularly useful when applied to patterns such as the stripe pattern, because the signal lines can be routed in a linear fashion in both the row and column directions without having to bend the signal lines. Bending the signal lines is undesirable because bent signal lines are effectively longer and therefore more resistive. Bent signal lines also require more area in the plane of the active matrix circuitry leaving less area for other circuit components.
However, when applied to other patterns in which differently colored pixels are located in the same rows and columns, this approach becomes impractical. This is because each differently colored pixel will have, preferably, a differently sized emitting area. As such, for example, the widths of the pixels may need to be adjusted to optimize for differently colored pixels in the same row and the heights of the pixels may need to be adjusted to optimize for differently colored pixels in the same column.
As shown in the prior art described above, signal lines are typically located between the emitting areas of each pixel. For example, in the row direction, a select line may be located between each of the emitting areas of two adjacent rows of pixels. Adjusting the size of the emitting areas in both the row and column direction would result in the circuit components and signal lines needing to be routed around the emitting regions of the pixels. However, routing the signal lines around the emitting regions of the pixels results in the signal lines becoming bent as describe above.